1. Field of the Invention
The present invention is generally in the field of electrical circuits. More particularly, the invention is in the field of power amplifiers.
2. Background Art
A power amplifier used in a wireless communication device, such as a cellular handset, is required to have a gain that matches the communication standard of the system under which the device operates and the type of signal modulation that the system uses. For example, a power amplifier used in cellular handset operating a system using a global system for mobile communications (“GSM”) communications standard and Gaussian minimum shift keying (“GMSK”) modulation requires a different gain than a power amplifier used in a cellular handset in a system using an enhanced data for global evolution (“EDGE”) communications standard and EDGE modulation. Thus, to enable a wireless communication device, such as a cellular handset, to operate in two different systems that use different modulation types, a power amplifier is required to provide an appropriate gain for each system and modulation type.
In one approach, a power amplifier having three amplifier stages achieves a low gain mode by closing a switch to connect the input and output of a power amplifier input stage together, thereby reducing the three-stage amplifier to a two-stage amplifier. In the above approach, a high gain mode can be achieved by opening the switch to provide three stages of amplification. However, the switch utilized in above approach typically requires a number of additional components, such as blocking capacitors and inductors, for proper implementation, which undesirably increases manufacturing cost.
Also, the switch can cause undesirable signal loss in the low gain mode, which can be offset by additional amplification. However, the additional amplification required to offset the signal loss can undesirably increase the noise level in the power amplifier. Also, as effective switch may require a separate semiconductor die for fabrication, which significantly increases manufacturing cost.
Thus, there is a need in the art for a cost-effective power amplifier that can selectively operate in a low gain mode or a high gain mode.